All images in a chain must share the same encryption type and password, otherwise you'll end up with a usability nightmare (potentially having to enter tons of different passwords just to access a single point-in-time within a long chain). Therefore, the edit-job wizard won't let you change a password on an existing backup job. You have to specify the encryption level (and password) at the time that you create the backup job. In your case, you'll have to delete your existing backup job (you can keep your images), and create a new, similar, backup job with encryption enabled (set a password).